1. Field of the Invention
The present invention relates to a sealed device, and a leak test method thereof and a manufacturing method thereof.
2. Description of Related Art
Previously, a leak test, which uses a bombing method, is performed to test a fluid-tightness (gas-tightness) of a sealed device (see, for example, Japanese Unexamined Patent Publication No. H11-305665 and Japanese Unexamined Patent Publication No. 2006-53106). In the bombing method, the sealed device is placed in a bombing device, and a gas is evacuated from the interior of the bombing device. Thereafter, a tracer gas, such as helium (He) gas, is supplied into the interior of the bombing device, and the sealed device is pressurized (bombed) from the outside thereof with the tracer gas for several hours. At this time, when the sealed device has a leak, the tracer gas leaks into the sealed device at a rate, which corresponds to a size of the leak. Upon completion of the bombing, the sealed device is removed from the bombing device and is placed in a vacuum chamber. Then, the interior of the vacuum chamber is depressurized, and the amount of the outwardly leaked tracer gas (the leak rate of the tracer gas) from the sealed device is measured. The fluid-tightness of the sealed device is evaluated based on the amount of the outwardly leaked tracer gas, which is measured in the above-described manner.
Another method of measuring the leakage of the sealed device, which is other than the bombing method, is described in, for example, Japanese Unexamined Patent Publication No. 2002-134164. In this method, a tracer gas, such as helium (He) gas, is previously sealed into an interior of the sealed device during manufacturing thereof. In this method, the tracer gas of a relatively large amount and a relatively high concentration is filled into the interior of the sealed device within a relatively short time period, so that this method is advantageous over the bombing method.
However, in some cases, an upper limit exists on the enclosable amount of the tracer gas, which can be enclosed in the interior of the sealed device, to limit a substantial deterioration of the performance of the heat exchanger. For example, in some sealed heat exchangers, a pressure of a non-condensable gas, such as a tracer gas, in the interior of the heat exchanger needs to be kept equal to or less than a predetermined pressure (e.g., 5 kPa) throughout a product lifetime thereof. Thus, the amount of the tracer gas, which can be enclosed in the sealed device in the manufacturing thereof, should be relatively small in view of the fact that non-condensable gases of the surrounding atmosphere gradually leak into the interior of the sealed device during a period of actual use of the sealed device. Thus, the sufficient amount of the tracer gas cannot be leaked from the sealed device to allow measurement of the amount of outwardly leaked tracer gas. Therefore, the leak test of the sealed device becomes difficult.
Also, it is conceivable to perform the leak test by measuring the outward leakage of an enclosed liquid or vaporized gas thereof from the sealed device (e.g., water or water vapor in the case of the heat exchanger having the water enclosed therein). However, the water or the water vapor does not allow measurement of a fine leak from the sealed device with a high sensitivity. Thus, in this case, it is difficult to perform the leak test on the sealed device, which needs to have the high degree of fluid-tightness.